The invention relates to motion control apparatus for controlling the relative motion of first and second movable members, and is particularly, although not exclusively, applicable to the control of the deceleration of a lift car on approaching a stop position at floor level.
Ideally, a lift car should be controlled to approach a stop position with a constant rate of deceleration so that there is a square law relationship between distance to go and the instantaneous speed of movement. In known control apparatus, the position of the lift car is sensed, by means for example of an electromagnetic position sensing device, and an electrical analogue of the distance to go to the stop position is generated which is used to control the speed of the lift car. However, practical difficulties occur upon approaching zero speed, in that both the actual speed and the analogue signals diminish towards zero, and the process of speed control requires accurate comparisons of small quantities, in order to produce an effective result. The basis of one particular difficulty is that the control of movement of the machinery involves both dynamic and static loads and many of the loads are variables. In accurately positioning a lift, it is necessary to dissipate the kinetic energies of rotating parts and to offset steady forces due to the mechanical load placed within the lift, and the process of deceleration involves not only a reduction of power level but also the realization of significant levels of braking torque. For variable voltage lift systems in particular, where the braking torque arises from the production of reversed current flow in the drive motor during the whole or part of the deceleration programme, it is critically important that the machinery continues to rotate in the required direction and that, in approaching the stop position, the stability of the system response should be assured.